Executive control of internal mentation (i.e. reflecting upon one's past, thinking about one's future, or engaging in other introspective cognitive processes) is critical for successful behavior. Failures to appropriately enhance or suppress attention to internal information form the basis of attentional lapses and memory failures in healthy young and old adults, and represent a hallmark of cognitive dysfunction in Alzheimer's disease. Despite its utmost significance, the neural basis of executive control of internal mentation remains poorly characterized and represents the primary objective of the present proposal. In a series of three main experiments, we aim to 1) examine the anatomy and function of the brain's default network, a brain system thought to play a major role in internal mentation, 2) investigate the neural underpinnings of sustained and transient top-down control of internal mentation separately from external attention and 3) examine the neural mechanisms facilitating rapid switching between external and internal (introspective) modes of attention. A functional magnetic resonance imaging (fMRI) meta-analysis will be first conducted to examine the overlap between the default network and several different introspective tasks to gain insight into the network's overarching function. Next, an fMRI experiment that manipulates demand for control (easy vs. hard trials) and the nature of the task-relevant information (external vs. internal trials) will examine whether similar prefrontal mechanisms bias attention to both introspective and external information. Finally, a separate experiment will examine the behavioral signatures of rapid switching between external and introspective modes of attention as well as the hierarchical neural mechanisms that support this ability. Brain regions that play a role in rapid switching within domains (i.e. switching between different tasks of a similar nature) will be compared to those regions that underlie rapid switching between domains (i.e. switching between internal and external tasks). Knowledge of the neural mechanisms that govern executive control of internal mentation will provide additional insight into the causes of successful and unsuccessful behavior in young adults, will help explain the mechanisms that underlie distractibility and memory failures in advanced aging and AD, and will facilitate the development of cognitive training and rehabilitation paradigms applicable to these populations.